The present invention relates to a semiconductor laser having a GaAs substrate provided with an etched channel.
Japanese Patent Disclosure No. 52-90280 discloses a channeled substrate planar-stripe (CSP) laser, which is a GaAs-GaAlAs semiconductor laser capable of a transverse mode oscillation. FIG. 1 shows the structure of the CSP laser. The CSP laser comprises an n-type GaAs substrate 1 provided with an etched channel 2, an n-type GaAlAs cladding layer 3, a GaAs active layer 4, a p-type GaAlAs cladding layer 5, an n-type GaAs cap layer 6, a p-type GaAs contact area 7 prepared by selective zinc diffusion, a p-side electrode 8, and an n-side electrode 9. In the region outside the channel 2, the n-type cladding layer 3 is thin enough for the optical field to penetrate the lossy substrate 1 and, therefore, the optical field is confined to the channel region. This optical field confinement provides stable fundamental transverse mode oscillation, if the channel 2 is as narrow as the higher modes are cut-off.
However, the conventional device shown in FIG. 1 gives rise to a serious problem. In the prior art, the heteroepitaxial layers for this device are grown by liquid phase epitaxy (LPE) on a substrate with etched channels, which are, for example, 4 .mu.m wide and 1 .mu.m deep. During the heat-cycle of the LPE, these channels suffer from deformation and are widened. This deformation is caused by the As-atom migration into the ambient gas as well as by the GaAs dissolution into the Ga-Al-As solution. These phenomena are called "mass transfer" and "melt-back", respectively. As a result, the channel width w is increased to 7 .mu.m or more. Therefore, it is very difficult to form narrow channels with high reproducibility by using the prior art.
Japanese Patent Disclosure No. 57-159084 discloses a V-channeled-substrate inner-stripe (VSIS) laser in which an n-type GaAs layer grown on a p-type GaAs substrate acts as a current-blocking-layer (CBL). The basic structure of this prior art is shown in FIG. 3. The VSIS laser comprises a p-type GaAs substrate 13, an n-type GaAs layer 14, which act as a CBL, a channel 21 etched into the CBL 14 to reach the substrate 13, a p-type GaAlAs cladding layer 15, a GaAlAs active layer 16, an n-type GaAlAs cladding layer 17, an n-type GaAs ohmic layer 18, and electrodes 19 and 20. The heteroepitaxial layers 15-18 are grown by LPE successively. The transverse mode stabilization mechanism of the VSIS laser is equal to that of the CSP laser and, therefore, the optical field is restricted in the region of the channel. In the VSIS laser, however, the current is also restricted in the region of the channel because the current is blocked by the CBL outside the channel. Thus, the current width u shown in FIG. 3 as well as the channel width w is an important parameter for determining optical characteristics of the VSIS laser. The heteroepitaxial layers 15-18 are grown by LPE on the substrate with etched channels 21 as shown FIG. 4. However, during the LPE, the channels are deformed by mass transfer and melt-back and, therefore, the width w and u are increased as shown in FIG. 5. The larger w gives the narrower lateral beam divergence. If the width w becomes much larger than the width u, then the optical field is influenced by gain guiding rather than index guiding. As this channel deformation is not uniform within a wafer, the reproducibility of the optical characteristics is very poor even among the devices fabricated from the same wafer.
Japanese Patent Disclosure No. 56-40292 discloses a separated multicladding layer (SML) laser. The SML laser has a CBL like the VSIS laser, but the CBL in the SML laser is not deformed during the LPE. FIG. 6 is a cross sectional view showing the SML laser. In the SML laser, a GaAlAs layer 23 acts as both CBL and cladding layer. As Ga.sub.l-x Al.sub.x As (0.1.ltoreq.x.ltoreq.1) does not suffer from mass transfer nor melt-back, the channel 31 is not deformed during the LPE. However, the LPE growth on the side surface of the GaAlAs layer (CBL) 23, which defines a channel 31, is very difficult because of an oxide film on it, which is easily formed by air-exposure. As a result, epitaxial layers 25-28 frequently fail to grow as shown in FIG. 7, which is described in Japanese Patent Disclosure No. 57-93593.
It is seen that the SML laser shown in FIGS. 6 and 7 comprises an n-type GaAs substrate 22, an n-type Ga.sub.l-x Al.sub.x As (0&lt;x&lt;0.1) optical guide layer 24, an n-type GaAlAs cladding layer 25, a p-type GaAlAs active layer 26, a p-type GaAlAs cladding layer 27, a p-type GaAs cap layer 28, electrodes 29, 30, and the channel 31 in addition to the p-type GaAlAs layer (CBL) 23 mentioned above.